Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). Therein, a portion of an engine's cylinders may be disabled during selected conditions defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling.
Further improvements in fuel economy can be achieved in engines configured to vary the effective displacement of the engine by skipping the delivery of fuel to certain cylinders in an indexed cylinder firing pattern, also referred to as a “skip-fire” pattern. One example of a skip-fire engine is shown by Tripathi et al. in U.S. Pat. No. 8,651,091. Therein, an engine fuel controller may continuously rotate which particular cylinders are fueled, which cylinders are skipped, and how many cylinder events the pattern is continued for. By skipping fuel delivery to selected cylinders, the active cylinders can be operated near their optimum efficiency, increasing the overall operating efficiency of the engine. By varying the identity and number of cylinders skipped, a large range of engine displacement options may be possible.
However the inventors herein have identified potential issues with such engine systems Specifically, in the case of a boosted engine, turbocharger performance may be degraded when one or more cylinders are deactivated. This is because the distribution and frequency of exhaust pulses released from the active engine cylinders may affect the efficiency of the turbine(s). In addition, the reduced exhaust volume associated with cylinder deactivation may adversely affect turbo charger efficiency. As an example, based on the firing of the active cylinders, exhaust pulses may be directed to different regions of a turbine, or different turbines altogether, resulting in insufficient turbine spin-up and increased turbo lag. Consequently, transient performance of the turbocharger may be degraded. Further, the load range for a given cylinder pattern of the selective deactivation may be limited. Further still, long delays between exhaust pulses can allow the turbine to slow down, and potentially go into compressor surge. The inventors herein have recognized that on some engines, there may be some flexibility remaining to further optimize the cylinder pattern in view of turbocharger performance.
In one example, the above issues may be at least partly addressed by a method of operating an engine comprising: in response to a boost demand, selectively deactivating a cylinder pattern of individual cylinder valve mechanisms; the cylinder pattern selected to direct exhaust from active cylinders into one scroll of a multi-scroll exhaust turbine. In alternate examples, the cylinder pattern may be further selected to direct exhaust from active cylinders into one turbine of a multi-turbine engine system (such as a twin-turbo engine system). In this way, a cylinder pattern may be selected during selective cylinder deactivation where the exhaust pulses of the active cylinders improve the turbine response.
As an example, a boosted engine system may include a single twin-scroll turbine. In response to a low engine load condition, an initial set of deactivated cylinder patterns may be selected based on engine load. The selection of cylinder patterns based on engine load may include selecting a number of cylinders to deactivate and a number of cylinders that will continue firing, the number of deactivated cylinders increasing with decreasing engine load. The initial set of patterns may be further modified based on engine NVH constraints, for instance by removing engine patterns which degrade NVH from the initial set. Under some conditions, such as during a boost demand, the modified set of cylinder patterns may be further modified based on their effect on turbine efficiency. This may include selecting a cylinder pattern from the initial set that also improves turbine efficiency, such as by directing exhaust pulses from active cylinders towards only one of the two scrolls of the turbine, for example to only the inner scroll or only the outer scroll. In alternate examples, cylinder patterns that degrade turbine efficiency may be selected out and a remaining cylinder pattern may be applied in the presence of the boost demand. As such, the cylinder pattern selected for improving turbine efficiency may be based on the specific configuration of the boosted engine. Thus, while the depicted example suggests selecting a cylinder pattern that directs exhaust pulses of active cylinders to a single scroll, in embodiments where the engine has multiple turbines, a cylinder pattern may be selected that directs exhaust pulses to a single turbine. In this way, during cylinder deactivation conditions where boost is requested, a cylinder pattern of deactivated/active cylinders may be selected, whenever possible, to improve turbine performance. By choosing a cylinder pattern wherein the distribution and frequency of exhaust pulses from the active cylinders are advantageously used to improve turbocharger performance, enhanced boost performance may be achieved along with cylinder deactivation benefits. For example, by selectively directing exhaust pulses from the active cylinders towards a single turbine, or a single turbine scroll, sufficient exhaust may be provided to the selected turbine to enhance turbine operation despite low exhaust volumes. As such, this allows for improved boosted engine performance in low-to-mid loads with cylinders deactivated. Overall, cylinder deactivation benefits, such as improved fuel economy, can be extended to a wider range of boosted engine operating conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.